Switched-mode power supplies are generally used in applications in which there is a need for an accurate and versatile control of the output voltage and current of a device producing direct voltage. The principle of a switched-mode power supply is to feed electric power by pulses to an inductive component, which has an energy-storing magnetic field. By means of a suitable discharge coupling, the stored energy is discharged into an output voltage and current, the values of which can be regulated by changing the properties of the input by pulses. Many methods of regulation are known, and among the most common of them is a method in which the output voltage and/or current is measured and a feedback signal indicating the measurement result is transmitted to a circuit which produces the switching pulses regulating the operation of the switching transistor.
FIG. 1 shows a simplified prior art flyback-type switched-mode power supply 1 with a two-coil transformer 2 for transmitting the electric power from the primary side to the secondary side and an opto-isolator 3 for transmitting the feedback signal from the secondary side to the primary side. The electric power comes to the primary side from the AC power supply 4 via a rectifier 5. A capacitor 6 reduces the disturbances of the rectified input voltage and prevents the high-frequency disturbances created by the switched-mode power supply from proceeding towards the AC power supply. The PWM (Pulse Width Modulation) controller 7 gives switching pulses to the base of the switching transistor 8, whereby the primary current runs through the primary coil 2a during each switching pulse. On the secondary side, the diode 9 and the capacitor 10 produce direct voltage to the output line 11 from the electric power transferred to the secondary coil 2b with the known flyback principle (see e.g. Horowitz P., Hill W.: "The Art of Electronics", 2nd ed. ISBN 0-521-37095-7, Cambridge University Press, New York, USA, 1989, p. 355-368).
On the secondary side, an operation amplifier 12 compares the portion of the output voltage produced by the voltage division coupling 13 with the voltage produced by the reference voltage generator 14. If the output voltage rises too high, the signal produced by the operation amplifier 12 lights the light-emitting diode of the opto-isolator 3, whereby the transistor which is the other component of the opto-isolator starts to conduct. Thus the simplified control principle of the switched-mode power supply 1 is that the primary side feeds a certain maximum amount of electric power to the transformer, until it receives from the secondary side a signal indicating that the output voltage is rising too high, whereby the primary side starts to reduce the input of electric power. Often the switched-mode power supply also includes a power limiting coupling, whereby too high a value of the output current causes a signal which is transmitted via the opto-isolator 3 or another signal isolator beside it to the primary side, where it limits the average electric power to be fed to the transformer. One known output limiting coupling comprises a series resistor on the output current path, in which resistor the voltage loss is equal to the product of the resistance of the series resistor and the output current. The voltage loss in question is measured by an operation amplifier, which gives a control signal to the primary side generally in the same manner through the opto-isolator as the operation amplifier 12 which measures the output voltage in the switching shown in FIG. 1.